Variable oscillation period seismometers



Aug. 3, 1965 Filed 001;. 12. 1962 i P. L. WILLMORE ETAL 3,199,072 i VARIABLE OSCILLATOR PERIOD SEISMOMETERS 2 Sheets-Sheet 1 W44; FIG. 1 WM 91% 3, 1965 P. L. WILLMORE ETAL 3,199,072

VARIABLE OSCILLATOR PERIOD SEISMOMETERS Filed Oct. 12. 1962 2 Sheets-Sheet 2 z wam 4: 27%

United States Patent 3,199,072 VARIABLE OSCILLATHUN PERISD SEISMGMETERS Patrick Lever Willrnore, Edinburgh, Scotland, and David Vivian Connell, London, England, assignors to Hilger & Watts Limited, London, England, a British company Filed Oct. 12, 1962, Ser. No. 230,125 12 Claims. (Cl. 340-47) This invention. relates to seismometers. These instruments are known in many different forms, which commonly rely upon the movement induced in a suitably suspended mass by a vibration received, to produce a signal or record of that vibration. The natural period of oscillation of the suspended mass determines the longest period of vibration to which it can satisfactorily respond.

Seismometers commonly comprise a mass suspended and constrained upon resilient support means, so that movement can only take place along a fixed line. The natural period of oscillation of the mass depends on the size of the mass and on the design. and adjustment of the resilient support means. It has now been found that if an additional resilient device is connected between a fixed part of the seismometer and the mass it can be arranged to act upon said mass so as to alter itsperiodv of oscillation on the resilient support means in dependence on the construction and adjustment of said additional resilient device.

Thus, according to one aspect of the present invention, there is provided a seismometer comprising a mass suspended upon resilient support meanswhich constrain the mass to move along a line, and a resilient device comprising a resilient element attached at one end to a fixed part of the seismometer so as to be fixed relative to the axis of the movement of the mass and a force transmission. element secured between the other end of the resilient element and the mass, the resilient element transmitting a force, which can be varied by adjustment means, through the force transmisison element to the mass when the mass is. displaced: from its equilibrium position so as to alter its period of oscillation in dependence upon the adjustment of the resilient clement, adjustable damping means being provided, the adjustment of which can be co-related to the altered, period of oscillation of the mass.

Such independent means for varying the period of oscillation of'the mass permits. the use of types of resilient support means which would not otherwise be compatible with; a. given size of mass to produce a desired period of oscillation and, therefore, can offer advantages in such respects as cost, portability, standardization and increased flexibility of operation.

However, there may be cases where the instrument is to perform a set function in operation for which the resilient device need not be adjustable. According to anothen aspect of the invention then, there is provided a seismometer: comprising amass suspended upon resilient su port means which constrain the mass to move along a line, and additionally to said means, a resilient device comprising a resilient element attached at one end to a fixedpart of the seismometer so as to be fixed relative to the axis of movement of the mass and a force transmission element securedv between the other end of the resilient element and the mass, the resilient element transmitting a force to the mass when the, force transmission element is displaced by the. movement of the mass from its equilibrium position, which force acts upon the mass either inthe direction of movement to increase its period of oscillation or in the opposite direction tosaid movement to decrease its period of oscillation.

One formof the invention willnow be more particularly described with reference to the accompanying drawings in which FIGS. 1 and 2 show, respectively, a partly sectioned elevation and a sectional plan on the line II II in FIG. 1 of a known Willrnore pattern seismometer adapted according to the present invention.

The type of instrument generally referred to as a Willmore pattern seismometer has been described in the Monthly Notices of the Royal Astronomical Society, Geophysical Supplement, volume 6, page 129 ff. The particular instrument shown in the accompanying drawings. is based upon the one-second period seismorneter described in that source.

Referring to the drawings, there may be seen within an outer envelope 4 sealing the working parts from the atmosphere, a rigid instrument frame 2. A permanent magnet 6 forms the oscillating mass of the instrument and its motion is constrained by three lower spokes 8 (only one of which is shown) and two upper spokes 10 (only a part of one of which is shown). A shaft 12 extending axially from the magnet terminates in arms 14 which are connected to a fixed boss 16 through a lifting spring system 18 comprising triangular plan form leaf springs 19. A cylindrical coil 20 is fixed to the rigid frame 2 and projects. between the poles of the magnet mass 6. For the transportation of the instrument, the boss 16 is internally threaded and carries a clamping screw 22 which can be screwed down to bring two pairs of frustoconical mating surfaces 24a, 24b and 26a, 2611 into engagement and so hold the magnet against movement.

To adjust the period of oscillation of the magnet 6, a sixth spoke 23: is provided which is clamped at one end to an extension arm 3t) on the magnet and is pivoted at its other end to the apex of a triangular form leaf spring 32- about twice the length of the spoke 28 and extending transversely thereto. This spring 32, as well as the leaf springs 19, is preferably made of a material having a constant modulus of elasticity over a wide temperature range. The broader end of the spring 32 is clamped to a gear quadrant 34 pivoted at 36 to the instrument frame 2. An adjustment gear 38 meshes with the teeth of the quadrant 34. The gear 38 is fixed to one end of and can be operated by shaft 40, the other; end of which extends vfrom the envelope 4' of the instrument to permit adjustment where the interior of the instrument is sealed. Thus, since the magnet is already restrained against turning by the five spokes 8, 10, operation of the shaft 4% to rotate the quadrant 34- will deflect the spring 32 and produce a compressive or tensile force in the sixth spoke 28. The spoke 28 is so arranged that, when the magnet is in an equilibrium position, this spoke extends perpendicularly to the axis of oscillation of the magnet.

The effect of a compressive force in the spoke 28 is to produce reacting forces in the other five spokes. The oscillation of the magnet mass, as it moves from its equilibrium position, will produce components of these forces acting in the direction of movement of the magnet from its equilibrium position, the result of which will be to lengthen the period of oscillation. Conversely, when a tensile force is applied to the spoke 28, therewill be components of these forces acting in the opposite direction of the movement of the magnet which will tend to restore the magnet to its equilibrium position and so reduce the period of oscillation. In the particular embodiment described it has been found possible to vary the period of oscillation of the magnet from /2 second to several seconds. 7

By suitable design, it' may be possible to produce an increase in the period of oscillation when the spoke 28 is put in tension and a decrease when it is put in com pression.

To use the instrument over the range offrequencies obtainable, it is necessaryto make some provision for a corresponding adjustment of the damping of the vibratory system. In the particular embodiment described, the coil 2t) is used for this purpose and a series of tappings 44 on its windings permit such adjustment, any one or more of which may be connected in series or in parallel to a load resistor (not shown), one or more further tappings being used in known manner for the coupling of a galvanometer or other detector to the coil to measure the output from the instrument.

It will readily be appreciated that the construction illustrated, which is arranged to be adjustable in the field, could be modified, where this facility is not required, to provide a form of instrument having a period of oscillation which can be altered by a relatively simple workshop operation. In fact, the basic form of instrument described could provide a standard pattern for manufacture of a range of instruments having ditferent frequencies, only the additional resilient device and the damping means being particular to any particular member of the range. These parts could be detachably fixed in place so as to permit modification of the instrument to give a different frequency response if required.

What we claim and desire to secure by Letters Patent is:

l. A seismometer comprising, in combination, an instrument body, a mass member, resilient support means connected between the body and said member to constrain said member to move along a line about an equilibrium position, an additional resilient device comprising an elongate elastic element and an elongate compression force transmission element, said elements being secured to each other at one of their respective ends, the other end of the compression force transmission element being attached to said member and the other end of the elastic element being adjustably attached to said body, the elastic element being capable of transmitting a compressive force through said transmission element to the mass element when it is displaced from its equilibrium position so as to increase its period of oscillation in dependence upon the adjustment of said elastic element, and adjustable damping means adjacent said member regulatable in co-relation to the altered period of oscillation thereof.

2. A seismometer according to claim 1 wherein the elastic element is a leaf spring of triangular form, the apex thereof forming the end attached to the force transmission element and the base thereof forming the end attached to said body, said spring being disposed in a plane parallel to the instantaneous path of the mass member when said member is in its equilibrium position.

3. A seismometer according to claim ll arranged for vertical operation having a plurality of lifting springs connected between said body and said member to hold said member against the force of gravity.

4. A seismometer comprising, in combination, an instrument body, a mass member, resilient support means connected between said body and said member to constrain said member to oscillate along a line path about an equilibrium position, an additional resilient device comprising a spring element and a linking element, one end of the spring element being attached to said body and the other end being attached to one end of the linking element, the other end of the linking element being attached to said member so that it transmits a force from the spring element to said member, the link element being angularly displaced by the movement of said member from its equilibrium position so that a component of said force acts along said line path to alter the period of oscillation of said member, the direction of said component being reversed as the member passes its equilibrium position.

5. A seismometer according to claim 4 wherein at least a part of said device is detachably mounted in its operative position so as to be replaceable by a substitute element of different physical characteristics to alter the period of oscillation of said member.

6. A seismometer comprising, in combination, an instrument body, a mass member, resilient support means comprising five elastically flexible spokes connected three to one end of said member and two to the other end thereof to constrain said member to move along a straight line path about an equilibrium position, an additional resilient device connected between said body and said member by adjustable means, said device being arranged to act on said member when it is displaced from said equilibrium position so as to alter its period of oscillation in dependence upon the adjustment of said means, adjustable damping means being provided for said member, the adjustment of which can be co-related to the altered period of oscillation thereof.

7. A seismometer according to claim 6 wherein the mass member comprises a permanent magnet and wherein a dampin coil is fixed to said body adjacent the magnet, said coil being provided with a series of tappings for regulation of the damping effect thereof.

8. A seismometer according to claim 6 wherein said device comprises a spring element and :1 spoke, said spoke connecting one end of said spring element to said member, a control member extending externally from said body being adjustable thereon, said control member being connected to the other end or" said spring, said adjustment of said control member altering the force applied by said spring element to said spoke.

9. in a seismometer, a mass member supported by resilient means which constrain the movement of said member to a linear oscillation about an equilibrium position, and an additional resilient device including energy storage means and a force transmission element having one end fixed to said member and an opposite end fixed to said energy storage means, said element being angularly displaceable by the movement of said member and lying perpendicular to the instantaneous path of said member when it is in said equilibrium position.

It In a seismometer according to claim 9, said energy storage means comprising a leaf spring, and adjustment means mounted on the instrument body and attached to an end of said leaf spring remote from its connection to said member, the adjustment of said means resiliently deforming said leaf spring to alter the force transmitted to said member through said element.

11. In a vertical component seismometer, a mass member supported by resilient means which constrain the movement of said member to a linear oscillation about an equilibrium position, an additional resilient device comprising first means for energy storage, second means of elongate form for force transmission and third means for mean energy storage adjustment, said first means being spaced from the mass member and being connected at one end by said second means to the mass member, said third means forming a connection between the other end of said firstmeans and a fixed part of the instrument, said device acting on said member when it is displaced from said equilibrium position so as to alter its period of oscillation in dependence upon the adjustment of said third means, said second means being angularly movable with the displacement of said member and the force transmitted by said second means being directed perpendicular to the instantaneous path of said member when said member passes through its equilibrium position.

12. In a vertical component seismometer, a mass member, resilient lifting means connected to the mass member to hold it against the force of gravity, resilient constraint means connected to the member to restrict its freedom of movement to a linear oscillation about its equilibrium position, an additional resilient device comprising first means for energy storage, second means of elongate form for force transmission and third means for mean energy storage adjustment, said first means being spaced from the mass member and being connected at one end by said second means to the mass member, said third means form- 5 6 ing a connection between the other end of said first References Cited by the Examiner means and a fixed part of the instrument, said device act- UNITED STATES PATENTS mg on said member when it 1s displaced from the equilibrium position so as to alter its period of oscillation in 2,636,160 4/53 P? et 340 17 dependence upon the adjustment of said third means, 5 6/56 Mlllmgton 340 '17 said second means being angularly movable with the 2788512 4/57 Relchert 340-17 displacement of said member and the force transmitted by said second means being directed perpendicular to the BENJAMIN BORCHELT Prlmary Examiner instantaneous path of said member when said member CHESTER L. JUSTUS, SAMUEL FEINBERG, passes through its equilibrium position. 10 Examiners. 

1. A SEISMOMETER COMPRISING, IN COMBINATION, AN INSTRUMENT BODY, A MASS MEMBER, RESILIENT SUPPORT MEANS CONNECTED BETWEEN THE BODY AND SAID MEMMBER TO CONSTRAIN SAID MEMBER TO MOVE ALONG A LINE ABOUT AN EQUILIBRIUM POSITION, AN ADDITIONAL RESILIENT DEVICE COMPRISING AN ELONGATE ELASTIC ELEMENT AND ELONGATE COMPRESSION FORCE TRANSMISSION ELEMENT, SAID ELEMENTS BEING SECURED TO EACH OTHER AT ONE OF THEIR RESPECTIVE ENDS, THE OTHER END OF THE COMPRESSION FORCE TRANSMISSION ELEMENT BEING ATTACHED TO SAID MEMBER AND THE OTHER END OF THE ELASTIC ELEMENT BEING ADJUSTABLY ATTACHED TO SAID BODY, THE ELASTIC ELEMENT BEING CAPABLE OF TRANSMITTING A COMPRESSIVE FORCE THROUGH SAID TRANSMISSION ELEMENT TO THE MASS ELEMENT WHEN IT IS DISPLACED FROM ITS EQUILIBRIUM POSITION SO AS TO INCREASE ITS PERIOD OF OSCILLATION IN DEPENDENCE UPON THE ADJUSTMENT OF SAID ELASTIC ELEMENT, AND ADJUSTABLE DAMPING MEANS ADJACENT SAID MEMBER REGULATABLE IN CO-RELATION TO THE ALTERED PERIOD OF OSCILLATION THEREOF. 